This invention is generally directed to a method for providing a strongly adherent and durable polycrystalline diamond film on scribing wheels by chemical vapor deposition which includes pretreating the wheels.
Due to the need to produce various size and shapes of glass in different applications, glass cutting is an essential operation in glass manufacturing and processing. Because of the brittle nature of glass, the method and the tool used for glass cutting are quite different from those used for cutting other more plastic materials such as metal, metal alloys or metal composites. In cutting the latter, actual pealing or removal of materials during cutting is involved. Metal cutting tools have sharp cutting edges at the corners or tips of the tool bodies which remove or peal a portion of the metal surface. These types of tools, however, are not suitable to cut glass or other similar glassy materials like ceramics or silicon because those glass materials are brittle and susceptible to chipping and breaking if cutting is attempted with conventional, metal cutting tools.
The standard way of glass cutting is by scribing a line over the surface of glass and subsequently bending along the line to separate the object. The tools for scribing the line can be of any sharp pointed hard objects or, more commonly, a scribing wheels. When the sharp edge of the scribing wheel is pressed and rolled over the glass, it creates high compressive stress on the surface resulting in subsurface tensile cracking or tearing of the glass. A narrow crevice is formed underneath the score line as shown in FIG. 1. When a bending moment is applied to both sides of the crevice, a strong tension stress is developed at near the bottom of the crevice causing it to cleave and propagate downward and eventually separate the piece. In this case, little or no material is removed from glass during cutting, and because of the continuous change in the contact surface of the tool, its temperature remains relatively low (less than 150.degree. C.).
Glass scribing wheels presently employed in the glass manufacturing industry are made of cobalt-cemented tungsten carbides (Co--WC). These wheels have reasonable hardness and wear resistance, but their useful lifetime is limited to a few hours of cutting. With use, severe wear occurs at the scribing edge around the circumference of the wheel so that it is necessary to frequently replace the wheel which results in substantial downtime and reduction in productivity.
Diamond is known to exhibit many excellent mechanical properties including superb hardness, low friction coefficient, high wear resistance, and high thermal conductivity which are ideal for making cutting tools. In fact, small diamond stones have been used for various cutting tools. However, the volume of consumption and associated costs of using pure natural diamond are prohibitive for industrial uses. Due to the recent advance in diamond synthesis, tools for cutting metals have been fabricated using such synthetic diamond. For example, metal cutting tools have been fabricated from sintered synthetic diamond powder, or by providing a diamond coating on the tool. U.S. Pat. No. 4,884,476 to Okuzumi, et al. discloses prefabricating thick diamond films and then bonding them onto the surface of a machining tool. The prefabrication of the diamond film and the fact that after applying the film to the tool it needs to be reground to proper tool shape make the process expensive. A more desirable way to apply a diamond coating to tools is by using chemical vapor deposition (CVD) techniques and diamond films have been applied to the surface of a drill, insert, or end mill used in the cutting using this technique.
In order to improve the adhesion between a diamond coating and the substrate, various techniques have been proposed. These include plasma ion etching, diamond powder scratching, and strong acid etching. The latter is exemplified in U.S. Pat. No. 4,731,296 to Kikuchi, et al. It discloses a method for improving the bond strength between a CVD applied diamond coating and a tungsten carbide-based material cutting tool for metals by subjecting the tungsten carbide tool to an aqueous solution of a strong acid such as nitric acid prior to providing the diamond coating. The acid removes cobalt present in the tungsten carbide as a binder from the surface region. The first two methods are unsuitable for use with Co--WC because they indiscriminately remove the tungsten carbide as well as the cobalt. The strong acid etching process is undesirable because it can cause excessive removal of the cobalt and damage the integrity of the bulk or the tool material in the surface region.
The method of the present invention provides a method of selectively and controllably removing undesirable materials, like oxides or cobalt, which may be present on the surface of the wheel and enhances the diamond nucleation of the CVD applied diamond film. Both of these aspects of the invention process contribute to the excellent diamond film adhesion and wear resistance of the present invention diamond coated scribing wheel.